Magnetic memories are often used in storing data. One type of memory element currently of interest utilizes magnetoresistance of a magnetic element for storing data. FIGS. 1A and 1B depict conventional magnetic elements 1 and 1′. The conventional magnetic element 1 is a spin valve 1 and includes a conventional antiferromagnetic layer 2, a conventional pinned layer 4, a conventional spacer layer 6 and a conventional free layer 8. The conventional pinned layer 4 and the conventional free layer 8 are ferromagnetic. The conventional spacer layer 6 is nonmagnetic. The conventional spacer layer 6 is conductive. The antiferromagnetic layer 2 is used to fix, or pin, the magnetization of the pinned layer 4 in a particular direction. The magnetization of the free layer 8 is free to rotate, typically in response to an external field.
The conventional magnetic element 1′ is a spin tunneling junction. Portions of the conventional spin tunneling junction 1′ are analogous to the conventional spin valve 1. Thus, the conventional magnetic element 1′ includes an antiferromagnetic layer 2′, a conventional pinned layer 4′, an insulating barrier layer 6′ and a free layer 8′. The conventional barrier layer 6′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 1′.
Depending upon the orientations of the magnetizations of the free layer 8 or 8′ and the pinned layer 4 or 4′, respectively, the resistance of the conventional magnetic element 1 or 1′, respectively, changes. When the magnetizations of the free layer 8 and pinned layer 4 are parallel, the resistance of the conventional spin valve 1 is low. When the magnetizations of the free layer 8 and the pinned layer 4 are antiparallel, the resistance of the conventional spin valve 1 is high. Similarly, when the magnetizations of the free layer 8′ and pinned layer 4′ are parallel, the resistance of the conventional spin tunneling junction 1′ is low. When the magnetizations of the free layer 8′ and pinned layer 4′ are antiparallel, the resistance of is the conventional spin tunneling junction 1′ is high.
In order to sense the resistance of the conventional magnetic element 1/1′, current is driven through the conventional magnetic element 1/1′. Current can be driven through the conventional magnetic element 1 in one of two configurations, current in plane (“CIP”) and current perpendicular to the plane (“CPP”). However, for the conventional spin tunneling junction 1′, current is driven in the CPP configuration. In the CIP configuration, current is driven parallel to the layers of the conventional spin valve 1. Thus, in the CIP configuration, current is driven from left to right or right to left as seen in FIG. 1A. In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 1/1′. Thus, in the CPP configuration, current is driven up or down as seen in FIG. 1A or 1B. The CPP configuration is used in MRAM having a conventional spin tunneling junction 1′ in a memory cell.
FIG. 2 depicts a conventional memory array 10 using conventional memory cells 20. Each conventional memory cell 20 includes a conventional magnetic element 1/1′, depicted as a resistor in FIG. 2. The conventional memory array 10 typically uses a spin tunneling junction 1′. The conventional array 10 is shown as including four conventional memory cells 20. Each memory cell 20 includes a conventional spin tunneling junction 1′ and a transistor 22. The memory cells 20 are coupled to reading/writing column selection 30 via bit lines 32 and 34 and to row selection 50 via word lines 52 and 54. Also depicted are write lines 60 and 62 which carry currents that generate external magnetic fields for the corresponding conventional memory cells 20 during writing. The reading/writing column selection 30 is coupled to write current source 42 and read current source 40 which are coupled to a voltage supply Vdd 48 via line 46.
In order to write to the conventional memory array 10, the write current 1w 42 is applied to the bit line 32 or 34 selected by the reading/writing column selection 30. The read current Ir 40 is not applied. Both word lines 52 and 54 are disabled. The transistors 22 in all memory cells are disabled. In addition, one of the write lines 60 and 62 selected carries a current used to write to the selected conventional memory cell 20. The combination of the current in the write line 60 or 62 and the current in the bit line 32 or 34 generates a magnetic field large enough to switch the direction of magnetization of the free layer 8′ and thus write to the desired conventional memory cell 20. Depending upon the data written to the conventional memory cell 20, the conventional magnetic tunneling junction 1′ will have a high resistance or a low resistance.
In conventional MRAM, the net magnetic field generated by the currents in the write line 60 or 62 and the bit line 32 or 34 is oriented at an off-axis angle of 3π/4 radians with respect to the easy axis of the free layer 8′. The easy axis of the free layer 8′ is the direction in which the magnetization of the free layer 8′ tends to reside in the absence of external fields. According to the “asteroid” switching threshold curve of the Stoner-Wohlfarth (SW) model for a single-domain magnet with uniaxial anisotropy, which has been confirmed experimentally many times, the total amount of applied current needed for switching is minimum when the off-axis orientation angle of the net generated magnetic field is 3π/4 radians.
When reading from a conventional cell 20 in the conventional memory array 10, the read current Ir 40 is applied instead. The memory cell 20 selected to be read is determined by the row selection 50 and column selection 30. The output voltage is read at the output line 44.
Although the conventional magnetic memory 10 using the conventional spin tunneling junction 1′ can function, one of ordinary skill in the art will readily recognize that there are barriers to the use of the conventional magnetic element 1′ and the conventional magnetic memory 10 at higher memory cell densities. In particular, the conventional memory array 10 is written using an external magnetic field generated by currents driven through the bit line 32 or 34 and the write line 60 or 62. In other words, the magnetization of the free layer 8′ is switched by the external magnetic field generated by current driven through the bit line 32 or 34 and the write line 60 or 62. The magnetic field required to switch the magnetization of the free layer 8′, known as the switching field, is inversely proportional to the width of the conventional magnetic element 1′. As a result, the switching field increases for conventional memories having smaller magnetic elements 1′. Because the switching field is higher, the current required to be driven through the bit line 32 or 34 and particularly through the write line 60 or 62 increases dramatically for higher magnetic memory cell density. This large current can cause a host of problems in the conventional magnetic niemory 10. For example, cross talk and power consumption would increase. In addition, the driving circuits required to drive the current that generates the switching field at the desired memory cell 20 would also increase in area and complexity. Furthermore, the conventional write currents have to be large enough to switch a magnetic memory cell but not so large that the neighboring cells are inadvertently switched. This upper limit on the write current amplitude can lead to reliability issues because the cells that are harder to switch than others (due to fabrication and material nonuniformity) will fail to write consistently.
Accordingly, what is needed is a system and method for providing a magnetic memory element which can be used in a memory array of high density, low power consumption, lower current, low cross talk, high reliability, sufficient read signal, while providing a short write time. The present invention addresses the need for such a magnetic memory element.